A molecular sieve is a hydrated porous crystalline aluminosilicate, which has a highly regular porous cage structure. NaY molecular sieve is a type of molecular sieve having a FAU topological structure, and is a molecular sieve having an ultra-cage structure formed by arranging β cages according to diamond structure. FAU type zeolite comprises X type and Y type, which are different from each other in the aluminum content, that is, silica-to-alumina ratios (SiO2/Al2O3) are different. Generally, an X type zeolite has a silica-to-alumina ratio of approximately 2.2-3.0, while a Y type zeolite has a silica-to-alumina ratio of 3-5.
The Y type zeolite is one of main species used as catalysts and adsorption and separation agents, and is applied in the process of petroleum catalytic cracking industry, an emerging oil refining technique which has been developed since 1960s. As a main active component of a catalytic cracking catalyst, the silica-to-alumina ratio of Y molecular sieve plays a dominating role in the catalytic cracking performance, the distribution of products, and the stability of catalyst. The Y molecular sieve having a high silica-to-alumina ratio has the advantages such as high catalytic activity and good stability, and greatly improves the aspect of the entire catalytic cracking industry. At present, the USY type zeolite used industrially is a Y zeolite having a relatively high silica-to-alumina ratio obtained by subjecting raw powder of Y zeolite to treatments such as chemical dealuminzation and physical dealuminzation. However, this post-processing dealuminzation method is energy intensive and complicated in practical application.
Generally, a NaY molecular sieve is prepared by a method using a directing agent under a hydrothermal condition. Specifically, a Y type molecular sieve seed crystal is first prepared under a high-alkalinity environment; this seed crystal is then added to a mixed gel system for NaY synthesis. The Y molecular sieve obtained by this method typically has a silica-to-alumina ratio of about 5.5. The direct method for hydrothermally synthesizing high-silicon Y type zeolite (SiO2/Al2O3>6) can avoid complicated procedure of post-processing, save a large amount of human resource and material resource, and reduce the pollution to the environment. At the meanwhile, there is a better catalytic effect due to a complete crystal structure and a uniform chemical distribution. Therefore, it has a great significance to explore a direct method for synthesizing high-silicon Y type zeolite.
In 1982, U.S. Pat. No. 4,333,859 disclosed a FAU polymorph designated as CSZ-3 having a silica-to-alumina ratio of greater than 6, which was obtained under a condition of using Cs+ having a larger ion radius together with seed crystals. The product had the following composition: 0.8-0.95Na2O:0.02-0.20Cs2O:Al2O3:5.0-7.0SiO2:2-10H2O.
In 1987, U.S. Pat. No. 4,714,601 disclosed a FAU polymorph designated as ECR-4 having a silica-to-alumina ratio of greater than 6, which was prepared by hydrothermal crystallization at 70-120° C. using bis(2-hydroxyethyl)dimethyl ammonium ions or other alkyl quaternary ammonium salts having larger sizes as a template agent. For the initial gel mixture of this material, the molar ratio of silica to alumina was 4-20, and the molar ratio of water to alumina was 100-400.
In 1990, F. Delprato (Zeolites, 10 (1990) 546-552) directly hydrothermally synthesized a Y type zeolite having a silica-to-alumina ratio of 6-10 by using crown ether for the first time, demonstrating the possibility for directly hydrothermally synthesizing a high-silicon Y type zeolite in the presence of an organic template agent. Subsequently, it was reported in a few articles and patents in this respect (Zeolites, 13 (1993) 122-127; U.S. Pat. No. 5,273,945; Zeolites, 15 (1995) 90-96). Although the crown ether has an excellent performance as a template agent, it is not suitable for practical production due to high price and high toxicity.
In 1990, U.S. Pat. No. 4,931,267 disclosed a FAU polymorph designated as ECR-32 having a silica-to-alumina ratio of greater than 6, which was obtained by using tetrapropyl- and/or tetrabutyl ammonium hydroxide as a template agent. For the reaction mixture for preparing this material, the molar ratio of silica to alumina was 9-36, and the molar ratio of water to alumina was 120-500.
In 1993, F. Delprato (Zeolites, 13 (1993) 122-127) also synthesized a high-silicon Y molecular sieve having a silica-to-alumina ratio of about 7 by using polyethylene oxide (PEO) as a template agent, wherein PEO having a molecular weight of 2000-8000 was advantageous for obtaining a well crystallized Y molecular sieve. The gel composition ratio of this system was 10SiO2:1Al2O3:2.4Na2O:xPEO:140H2O (the weight ratio of SiO2/PEO=3), and the high-silicon Y molecular sieve may be obtained by hydrothermal crystallization at 100° C. for 8 days.
In 1998, B. De Witte synthesized a high-silicon Y molecular sieve having a silica-to-alumina ratio of 6.2-6.6 by using inositol as a template agent (Microporous and Mesoporous Materials, 23 (1998) 11-22). The gel composition ratio of this system was 10SiO2:1Al2O3:2.7Na2O:0.5 inositol:140H2O, and the high-silicon Y molecular sieve may be obtained by hydrothermal crystallization at 110° C. for 8 days.
CN1736867A disclosed the preparation of a mesoporous Y molecular sieve by using 1-cetyl-3-methyl imidazolium bromide or 1-cetyl-3-methyl pyridinium bromide, a long-chain alkyl ionic liquid, as a template agent. It focused on the effect of the Y molecular sieve having a mesoporous structure on the catalytic activity, and did not relate to the content whether the silica-to-alumina ratio thereof was increased or not, compared to that of a Y molecular sieve synthesized by a conventional method without template.
The template agents previously used for synthesizing high-silicon Y molecular sieve are mainly quaternary ammonium salts, cyclic- or chain-type alcohols, ethers, etc. No NaY molecular sieve having a silica-to-alumina ratio of 6 or more and a high crystallinity is synthesized by using a less volatile short-chain alkylimidazolium ionic liquid as a template agent or a structure directing agent under a hydrothermal condition.